The present invention relates to a sheet finisher that conducts post-processing for a sheet and to an image forming system provided with the sheet finisher.
In the case of an image forming apparatus forming an image on a sheet at high speed as in an electrophotographic image forming apparatus, it is possible to provide an image forming system capable of meeting broad users needs, by connecting a sheet finisher having various sheet-processing functions to the image forming system.
For example, in Unexamined Japanese Patent Application Publication No. 2002-128384, there is disclosed an image forming system wherein an image forming apparatus has a sheet finisher having therein functions for a punching process, for binding process and for folding process.
The specification of U.S. Pat. No. 7,207,557 discloses an image forming system wherein there is arranged a common single-sheet processing device between an image forming apparatus and at least one type of a sheet finisher among plural types of sheet finishers.
In the case of an image forming system described in Unexamined Japanese Patent Application Publication No. 2002-128384, it is effective when it is installed in the environment where various types of users use it in various ways, as in offices, for example, because a sheet finisher is constructed to exhibit various functions for post-processing in a single device. Further, the sheet finisher of this kind is relatively compact, which is also effective for the office where space saving is required.
On the other hand, in the image forming system such as a short-run printing system, an image forming system does not always need to be equipped with all post-processing functions, and a certain specific post-processing function only can meet the needs sufficiently in many cases. Namely, when using as an image forming system such as short-run printing, a specific user uses a specific post-processing function only more frequently than in the occasion where various types of users use in various ways as in the case of using in the office.
In the aforesaid image forming apparatus, a single relatively compact sheet finisher has various post-processing functions to meet various usage patterns. However, if individual post-processing function is observed by specifying it, that post-processing function is not regarded to be at a sufficient level.
In recent years, an image forming apparatus of an electrophotographic method has come to be used in a short-run printing field. Namely, using the image forming apparatus equipped with the sheet finisher mentioned above makes it possible to achieve bookbinding of a print-on-demand system for making necessary number of sets only when they are needed.
In addition, no time is required for preparation of a printing plate which has been needed in conventional printing, whereby, enhancement of efficiency of bookbinding work and cost reduction therefore are greatly expected.
The image forming system described in U.S. Pat. No. 7,207,557 is an apparatus capable of meeting the aforesaid demands, and it is an image forming system with a structure wherein a single-sheet processing device representing one type of sheet finisher is connected on the side of a sheet-ejection portion of an image forming apparatus, and further, at least one type of sheet finisher among plural types of sheet finishers is connected to the single-sheet processing device.
Further, in the image forming apparatus, or in the image forming system wherein the sheet finisher is connected to the image forming apparatus, it is desired that the number of sheets processed per unit time (hereinafter referred to as productivity) is large. In the field of short-run printing, in particular, that demand is strong. The number of sheets processed by the image forming system is determined by the capacity of a sheet finisher in many cases, rather than by the capacity of an image forming apparatus.
Namely, in the sheet finisher, it is necessary to keep a sufficient space between continuous sheets (hereinafter referred to as space-between-sheets) because conveyance of a sheet is stopped temporarily for processing in many cases. As a measure for that purpose, a conveyance speed in the sheet finisher is made to be higher than that in the image forming apparatus. However, in the recent speeding up, the conveyance speed in the sheet finisher is approaching the limits, because the conveyance speed of the image forming apparatus is made to be higher.
Therefore, Unexamined Japanese Patent Application Publication No. 2003-54809 discloses a sheet finisher wherein a reversal conveyance section accepting plural stacked sheets is provided in the finisher, and plural sheets stacked on the reversal conveyance section are conveyed simultaneously. By conveying plural sheets simultaneously while they are superposed, it is possible to broaden the space-between-sheets as far as the superposed sheets, without enhancing the conveyance speed, namely, to improve productivity in the sheet finisher.
However, in the sheet finisher disclosed by Unexamined Japanese Patent Application Publication No. 2003-54809, sheets are compelled to be conveyed through a curved conveyance path (hereinafter referred to as a curved portion) while they are superposed. Therefore, a sheet conveyed through the inside of the curved portion is different from that conveyed through the outside of the curved portion in terms of a length of the conveyance path and of frictional force between a wall surface of the curved portion and a sheet. Due to this, sheet displacement is caused between the inside sheet and the outside sheet.
In the short-run printing field, qualitative request about positional accuracy in post-processing is strict, and conducting post-processing under the condition of this sheet displacement leads to occurrence of defective articles.
It is possible to provide an alignment mechanism such as a stopper member for correcting sheet displacement. However, in this case, the structure is complicated, resulting in cost increase, and a period of time for alignment is needed, making it impossible to achieve original objective to improve productivity in a sheet finisher.